 because the water will cling to it. It's less likely that if you have a big overhang in your shingles that it will run up, but... It'll creep up the back, right? It'll creep up the back. It's surface tension, so the water will come off the edge of the shingle, and it'll cling, and it'll run back up, and then it'll drip off when it hits the plywood, which is what you don't want, and it breaks there, or it's on your fascia board, and then you have water dripping down in behind your gutters, trapped,